1. Field of the Invention
The present invention relates to a radar system, and more specifically, to an FM-CW (frequency-modulated continuous wave) radar system.
2. Description of the Prior Art
In the FM-CW radar system, a relative velocity V of a target with respect to a signal transmitter and a range or distance D of the target from the signal transmitter are derived in the following manner:
A transmission signal, which is frequency-modulated using a modulating signal of a waveform having an ascending portion and a descending portion, is transmitted as a radio wave. Then, the transmitted radio wave is reflected by the target and received at the radar system. The received radio wave is formed into a received signal which is mixed with the transmission signal to produce a beat signal containing Doppler frequency components. Then, the beat signal is analyzed in frequency spectrum for selecting beat frequencies at regions thereof corresponding to the ascending portion and the descending portion of the modulating signal (hereinafter also referred to as "ascending-side beat frequency" and "descending-side beat frequency"), respectively. Using the selected beat frequencies, the distance D and the relative velocity V of the target are derived from equations (1) and (2) as follows, respectively: EQU D={C/(8.DELTA.F.multidot.fm)}.multidot.(fb1+fb2) (1) EQU V={C/(4f0)}.multidot.(fb1-fb2) (2)
wherein fb1 and fb2 represent the ascending-side beat frequency and the descending-side beat frequency, respectively, f0 represents a center frequency of the frequency-modulated transmission signal, .DELTA.F represents a frequency modulation width thereof, fm represents a modulating frequency, and C represents the velocity of light.
On the other hand, if a plurality of targets exist, it is essential to precisely derive and select ascending-side and descending-side beat frequencies for each target so as to achieve correct calculation of a relative velocity and a distance of each of the targets with respect to the signal transmitter.
For example, it is assumed that four targets are present. If these four targets are stationary or stopped, since a beat signal does not include Doppler frequency components, combinations of ascending-side and descending-side beat frequencies are simply in order of frequency magnitudes. This is shown in Table 1, wherein the targets are identified by NO. 1 to NO. 4.
TABLE 1 ______________________________________ Ascending-Side Descending-Side Target Beat Frequency Beat Frequency ______________________________________ NO. 1 fb11 fb12 NO. 2 fb21 (&gt;fb11) fb22 (&gt;fb 12) NO. 3 fb31 (&gt;fb21) fb32 (&gt;fb22) NO. 4 fb41 (&gt;fb31) fb42 (&gt;fb32) ______________________________________
As appreciated from Table 1, by selecting the ascending-side beat frequencies and the descending-side beat frequencies in order of frequency magnitudes of the beat signal, the information about the four targets can be derived using the foregoing equations (1) and (2).
On the other hand, if each of the four targets has a relative velocity with respect to the signal transmitter, since the beat signal includes the Doppler frequency components, it is possible that the order of the ascending-side beat frequencies differs from the order of the descending-side beat frequencies with respect to the four targets. Thus, on such an occasion, the correct combinations of the ascending-side and descending-side beat frequencies can not be established simply in order of frequency magnitudes as opposed to Table 1. This particularly occurs when the targets are located in proximity to each other. Accordingly, the correct information about relative velocities and distances of the respective targets can not be achieved based on the combinations of the ascending-side and descending-side beat frequencies determined simply in order of frequency magnitudes.